Sednoids, a term typically used to describe distant trans-Neptunian objects like Sedna, aren’t classified as dwarf planets mainly because they do not meet all of the criteria set forth by the International Astronomical Union (IAU) for such a classification. According to the IAU’s definition, for an object to be considered a dwarf planet, it must (1) orbit the Sun, (2) be massive enough for its gravity to have shaped it into a nearly round form, (3) not have cleared its orbital neighborhood of other debris, and (4) not be a natural satellite (moon).
Sednoids do satisfy the first and fourth criteria as they orbit the Sun and are not satellites. However, they generally do not display a shape that suggests hydrostatic equilibrium, which is often used as a basis for determining whether an object is “nearly round.” Many of these bodies, including Sedna, have not been observed in sufficient detail to definitively assess their shapes. Furthermore, their remote orbits and limited observational data make it challenging to confirm whether they have achieved the necessary mass and roundness associated with establishing themselves as dwarf planets.
Thus, until more detailed observations or additional criteria are considered for classifying distant objects in the solar system, Sednoids remain unclassified as dwarf planets.